The velocity dispersion profile of the remote dwarf spheroidal galaxy leo I: A tidal hit and run?

We present new kinematic results for 387 stars near the Milky Way satellite dwarf spheroidal galaxy Leo I. Spectra were obtained with the Hectochelle multiobject echelle spectrograph on the MMT, centered in the optical near 5200 Å. From 297 repeat measurements of 108 stars, we estimate the mean velocity error (1 σ) of our sample to be 2.4 km s^-1, with a systematic precision of <1 km s^-1. The final sample of 328 Leo I members gives a mean heliocentric velocity of 282.9 ± 0.5 km s ^-1 and a dispersion of 9.2 ± 0.4 km s^-1. The dispersion profile of Leo I is flat to beyond its classical "tidal" radius. We fit the profile to various equilibrium dynamical models. We strongly rule out all models where mass follows light. Anisotropic Sérsic+NFW models fit the dispersion profile well, but isotropic models are ruled out at a 95% confidence level. Inside a projected radius of ∼ 1040 pc, the mass and V-band mass-to-light ratio of Leo I from equilibrium models are in the ranges (5-7) × 10⁷ M⊙ and 9-14 (solar units), respectively. Leo I members outside a "break radius" of R_b ∼ 400" (500 pc) exhibit significant velocity anisotropy, whereas stars interior to this radius are consistent with an isotropic velocity distribution. We interpret the break radius as the tidal radius of Leo I at perigalacticon some 1-2 Gyr ago. This interpretation accounts for the complex star formation history of Leo I, population segregation within the galaxy, and Leo I's large outward galactocentric velocity. The lack of evident tidal arms in Leo I suggests that the galaxy may have been injected into its present highly elliptical orbit by a third body a few Gyr before its last perigalacticon. This scenario is plausible within current hierarchical structure formation models.